The treatment of an infected dental root is carried out by extracting the pulp using special instruments, then by shaping the root canal using successive drilling procedures, traditionally carried out with instruments of varying size and conicity. The final operation consists of filling the root canal.
The shaping of the root canal consists initially of enlarging the canal in its crown and medial parts to permit, as a second step, easier treatment of the apical part of the canal by mechanical cleaning the infected tissues.
An instrument designed for drilling dental root canals generally has a tapered rod fitted into a handle to permit it to be driven manually or mechanically and comprising, on at least part of its length, helicoidal flutes with a constant or non-constant pitch, and having at least one cutting edge.
Used in continuous rotation, this type of instrument may have a tendency to screw itself into the canal. Apart from the screwing action, another problem occurring in the production of instruments for drilling root canals is that of the strength and flexibility of the instruments. Indeed, when the instrument is too flexible it may bend or break before the practitioner has been able to complete the operation and when the instrument is too rigid, it follows the curvature of the dental root canal only with difficulty.
Numerous instruments have been developed in response to these problems. Document EP 1 361 831 describes an instrument for drilling dental canals comprising a base, a cutting section and a guiding section, the cutting section being defined by an envelope of a cylindrical or conical shape, the longitudinal axis of which coincides with the axis of rotation of the instrument. The cutting section has clearance zones disposed set-back with respect to the envelope, alternating with drilling zones disposed on said envelope.
This alternating arrangement of zones on the envelope and set back from the envelope makes it possible to reduce the risk of screwing the instrument into the dental canal. Furthermore, the axis of the cutting section can be offset with respect to the axis of the envelope. This makes it possible to deepen the clearance zones and to make the evacuation of the debris during treatment more effective.
Document U.S. Pat. No. 7,955,078 describes an endodontic instrument for preparation of dental root canals comprising a body shaped to turn about an axis of rotation. The body has a centre of mass which is not located on the axis of rotation of the instrument, thus giving the impression that the instrument is undulating (“swaggering”) when it is rotated. An instrument such as this has greater flexibility and thus makes it possible to follow the complex curves of a dental root canal most effectively.
However, in these two documents the axis of the active part is offset with respect to the axis of rotation of the instrument over the whole length of said active part and in particular the axis of the point of the instrument is offset. This can generate a beating motion of the point within the canal. It thus becomes difficult to ensure optimum dimensioning of the canal during treatment, in particular in its apical portion. Moreover, a point with its axis offset also has the disadvantage of pushing the debris back towards the apical portion rather than evacuating it towards the top of the canal.